This morning the OMC1 lock acquisition was failing because the OMC2 was resonant as the same time as OMC1 (see attached figure). Thus when reaching a resonance on OMC1, the power on the B1 photodiodes was reaching 3 mW, which (as expected) triggered the safety closing the fast shutter.
I reduced the temperature of OMC2 from 23.326 to 23.314°c and the OMC2 is no longer resonant at the same time as OMC1.